Electrostatic discharge protection structures for high speed technologies with mixed and ultra-low voltage supplies

ABSTRACT

An electrostatic discharge (ESD) protection circuit in a semiconductor integrated circuit (IC) having protected circuitry. In one embodiment, the ESD protection circuit includes a pad adapted for connection to a first voltage source of a protected circuit node of the IC, and a silicon controlled rectifier (SCR) having an anode adapted for coupling to the first voltage source, and a cathode adapted for coupling to a second voltage source. At least one capacitive turn-on device respectively coupled between at least one of a first gate of the SCR and the first voltage source, and a second gate of the SCR and the second voltage source.

CROSS REFERENCES

This patent application is a continuation-in-part of U.S. patentapplication Ser. No. 10/099,600, filed Mar. 15, 2002 (now U.S. Pat. No.6,768,616), which claims the benefit of U.S. Provisional Applications,Ser. No. 60/276,415, filed Mar. 16, 2001; 60/276,416, filed Mar. 16,2001; Ser. No. 60/276,424, filed Mar. 16, 2001; and Ser. No. 60/318,548,filed Sep. 11, 2001, the contents of which are incorporated by referenceherein.

FIELD OF THE INVENTION

This invention generally relates to the field of electrostatic discharge(ESD) protection circuitry and, more specifically, improvements forsilicon controlled rectifier (SCR) and NMOS circuits in the protectioncircuitry of an integrated circuit (IC).

BACKGROUND OF THE INVENTION

The ongoing advancements in integrated circuit (IC) technologies haveled to the use of lower supply voltages to operate the IC's. Lowersupply voltages help cope with a problem of hot carrier induced, limitedlifetime for the IC's. Designing IC's with lower supply voltagesrequires the use of very thin gate oxides. The thickness of the gateoxides influences the amount of drive current that is generated. Thethinner the gate oxide layer, the more drive current is generated, whichthereby increases the speed of the circuit. The gate oxides (e.g.,silicon dioxide) may have a thickness of less than 3 nanometers, andfurther advancements will allow the gate oxide thickness to scale downeven further. The lower supply voltages also allow the use of siliconcontrolled rectifiers (SCRs) with very low holding voltages (e.g.,1.5-2.0V) without introducing a risk of latch-up. The thin gate oxides,which are used in conjunction with low supply voltages, require extremelimitation of transient voltages during an ESD event.

A problem arises using the very thin gate oxides because the oxidebreakdown voltage is less than the junction breakdown voltage (e.g., 6-9volts) that triggers an ESD protection circuit, such as an SCR or NMOSdevice. For example, a grounded-gate SCR (GGSCR) may be used to provideESD protection for an (I/O) pad. The GGSCR has a junction breakdownvoltage between 6-9 volts, which provides the trigger current for theSCR. As advances in technology allow reduction of the thickness of theoxide thickness below 3 nanometers, the gate oxide is subject to damageat turn-on and high current clamping voltages greater than approximately4-6 volts.

Therefore, there is a need in the art for an ESD protection devicehaving a lower trigger voltage, as well as a lower holding and clampingvoltage that can protect the gate oxide from damage during turn-on andoperation.

SUMMARY OF INVENTION

The disadvantages heretofore associated with the prior art are overcomeby various embodiments of an electrostatic discharge (ESD) protectioncircuit in a semiconductor integrated circuit (IC) having protectedcircuitry. In one embodiment, the ESD protection circuit is capacitiveturn-on SCR (CTSCR), which includes a pad adapted for connection to afirst voltage source of a protected circuit node of the IC, and asilicon controlled rectifier (SCR) having an anode adapted for couplingto the first voltage source, and a cathode adapted for coupling to asecond voltage source. At least one capacitive turn-on device isrespectively coupled between at least one of a first gate of the SCR andthe first voltage source, and a second gate of the SCR and the secondvoltage source.

In a second embodiment, an ESD protection circuit having reducedparasitic capacitance (C_(ESD)) of the ESD device is provided.Specifically, a pad is adapted for connection to a first voltage sourceof a protected circuit node of the IC. An ESD protection device havingan anode and cathode are respectively coupled to the pad and a secondvoltage source. A capacitance reducing diode (C_(DIO)) is seriallycoupled in a forward conduction mode between the anode of the ESDprotection device and the pad, where the capacitance reducing diode hasa parasitic junction capacitance value that is less than a parasiticcapacitance value of the ESD protection device.

As is discussed below, these two embodiments, as well as other variousembodiments, provide ESD protection for the protected circuitry of anIC, such that the capacitance turn-on device provides a lower triggervoltage, as well as a lower holding and clamping voltage, which canprotect the gate oxide from damage during turn-on and operation in theESD event.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic block diagram representing an ESD protectioncircuit of an integrated circuit (IC) having an ultra low turn-onvoltage device;

FIG. 2 depicts a graph of current and voltage characteristics for an ESDprotection device;

FIGS. 3 through 19 depict schematic diagrams of various embodiments ofan ESD protection circuit incorporating the teachings of the generic ESDprotection circuit of FIG. 1;

FIG. 20 depicts a schematic diagram of an ESD protection circuit for anintegrated circuit (IC) having mixed supply voltages;

FIG. 21 depicts a schematic block diagram representing an ESD protectioncircuit of the present invention having reduced parasitic capacitance;

FIGS. 22 through 24 depict schematic diagrams of various embodimentsincorporating the teachings of the generic embodiment of FIG. 21;

FIG. 25 depicts a schematic diagram of the ESD protection circuit havingSCR turn-on diodes act as a Darlington transistor pump;

FIG. 26 depicts a schematic diagram of a temperature compensated triggerdevice of the ESD protection circuit 302;

FIG. 27 depicts a schematic diagram of a multi-fingered DTSCR ESDprotection device having current mirrored triggers for each DTSCRfinger;

FIGS. 28 to 30 depicts schematic diagrams of various embodiments of aSCR complementary input protection circuit;

FIG. 31 depicts a cross-sectional view of an SCR having a Zener diodetriggering device of the present invention; and

FIG. 32 depicts a schematic diagram of the ESD protection circuit havinga complementary SCR turn-on Darlington transistor pump.

To facilitate understanding, identical reference numerals have been usedwhere possible, to designate identical elements that are common to thefigures.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described with reference to CMOS devices.However, those of ordinary skill in the art will appreciate thatselecting different dopant types and adjusting concentrations allows theinvention to be applied to Bipolar, BiCMOS, SiGe/BiCMOS, and otherprocesses that are susceptible to damage caused by ESD. The presentinvention includes various embodiments of an ESD protection devicehaving a turn-on voltage, a holding voltage, and high current clampingcharacteristics such that ESD transient voltages will be properlylimited to not damage a gate oxide or other vulnerable semiconductordevice.

FIG. 1 depicts a schematic block diagram representing an ESD protectioncircuit 102 of an integrated circuit (IC) 100. The representationillustratively depicts the ESD protection circuit 102 coupled to aprotected node of the IC 100 and an IC pad 104. The pad 104 may be aninput pad, an output pad, or a supply pad, and is coupled to a lowvoltage turn-on (i.e., “triggering”) device 108 and an ESD protectiondevice 106, such as a SCR or NMOS device. The ESD protection device 106is coupled to ground 112. The ESD protection device 106 has a turn-onterminal (e.g., gate (not shown)) that is coupled to the triggeringdevice 108. A shunt path 110 may optionally be coupled between thetriggering device 108 and ground 112. The triggering device 108 and ESDprotection device 106 (e.g., SCR) together serve as a protection device102 for circuitry (not shown) on IC 100 also coupled to the pad 104.

In particular, the triggering device 108 and ESD protection device 106protect the IC circuitry from electrostatic discharges (ESD) that mayoccur at the pad 104. When turned on, the ESD protection device 106functions as a shunt to redirect any ESD currents from the pad 104 toground 112. The trigger device 108 turns on (i.e., “triggers”) the ESDprotection device 106 to quickly dissipate the current, and as suchavoid an over-voltage ESD condition, as is discussed in further detailbelow regarding each embodiment.

FIG. 2 depicts a graph of current and voltage characteristics 200 forthe ESD protection device 102 of the present invention. The graphcomprises an ordinate 202 that represents current characteristics of theESD protection device 102, and an abscissa 204 that represents voltagecharacteristics of the ESD protection device 102. The voltagecharacteristic is divided into three regions defined by particularvoltages. In particular, a first region 206 for a low supply voltage hasa voltage range of 0-1.5 volts. A second region 208 for the ESDprotection device 102 holding voltage range is between 1.5 and 6 volts.A third region 210 for an over-voltage condition has a range of voltagestransients capable of damaging the gate oxide of the ESD protectiondevice 102, such as between 6-9 volts.

The current and voltage (IV) characteristics for both prior art NMOS andSCR protection devices are respectively represented by curves 212 and214. Both prior art ESD protection devices have triggering voltagesoccurring in the over voltage range 210 (e.g., curve portions 220 onprior art curves 212 and 214), which may damage the gate oxide layer ofthe ESD protection device 102. The triggering voltage for both the SCRand NMOS protection devices is approximately the same in value (e.g.,7-9 volts). However, the holding voltage for the SCR device (1.5 to lessthan 5 volts) is less than the holding voltage of the NMOS protectiondevice (approximately 5 volts).

As will be discussed regarding the embodiments depicted in FIGS. 3-19,the inventive ESD protection devices 102 have low triggering and holdingvoltages that are below the gate breakdown voltage (i.e., 6-9 volts)that damages the gate oxides of the ESD protection device 102. Inparticular, the triggering voltages of both the inventive ESD protectiondevices 102 fall within a tolerable voltage range of 1.5-6 volts.Moreover, the holding voltage, which provides the minimum voltagerequired to maintain the ESD protection device in a conductive “on”state, is within a tolerable voltage range, such that damage to the gateoxide is also minimized. For example, the SCR protection devicerepresented by curve 218 has both a triggering and holding voltage rangebelow 6 volts. Similarly, the NMOS protection device represented bycurve 216 has a triggering voltage below 6 volts within a tolerablerange, while its high holding voltage is slightly above 6 volts.

FIGS. 3-19 depict schematic diagrams of ESD protection devices 106coupled to diode turn-on triggering devices 108 of the presentinvention. The ESD protection devices 106 in the embodiments of FIGS.3-19, are capable of triggering and protecting the IC circuitry at lowvoltages of approximately 1.5 to 6 volts, as shown by thecurrent/voltage (I/V) characteristics graph of FIG. 2.

FIG. 3 depicts a schematic diagram of a first embodiment of an ESDprotection device 302. In particular, FIG. 3 depicts a schematic diagramof a diode turn-on SCR (DTSCR) protection device 302 of the presentinvention. The DTSCR 302 comprises a diode turn-on (“triggering”) device308 and a SCR 306, which together serve as a protection device 302 forthe circuitry on an integrated circuit (IC) 100. The DTSCR protectiondevice 302 protects the IC circuitry from electrostatic discharges (ESD)that may occur at the pad 104, which is coupled to the IC circuitry.When turned on, the SCR 306 functions as a shunt to redirect any ESDcurrents from the pad 104 to ground. The diode turn-on trigger device308 turns on, that is “triggers”, the SCR 306 to avoid an over-voltageESD condition.

Referring to the schematic diagram of FIG. 3, the SCR protection device306 is illustratively represented as an NPN transistor T1 310 and a PNPtransistor T2 312, as is well known in the art. The emitter of the PNPtransistor T2 312 forms an anode 322 of the SCR 306, which is connectedto the pad 104. The collector of the PNP transistor T2 312 is connectedto a first node 336, which is also connected to the base of the NPNtransistor T1 310, as well as to one side of a resistor R_(sub) 341. Thefirst node 336 includes a first trigger gate G1 of the NPN transistor T1310. The other side of resistor R_(sub) 341 is connected to ground 112,which serves as the cathode of the SCR 306. The resistor R_(sub) 341represents an intrinsic substrate resistance in the base of the NPNtransistor T1 310 of the SCR 306, which is formed by local substrateties coupled to ground 112. Furthermore, the emitter of the NPNtransistor T1 310 is also connected to the grounded cathode 112. Asecond node 334 includes the base of the PNP transistor T2 312 and thecollector of a NPN transistor T1 310. The second node 334 also mayinclude coupling of an optional second trigger gate G2 for the PNPtransistor T2 312. For a detailed understanding of a layout andcross-sectional implementation of an illustrative SCR and respectivetrigger gates, the reader is directed to commonly assigned U.S. Pat. No.6,791,122, which is incorporated by reference herein in its entirety.

A shunt resistor 110 is also coupled from the first node 336 to ground112. The shunt resistor 110 is external to the SCR transistors T1 310and T2 312, and is provided in parallel to the intrinsic resistanceR_(sub) 341 of the P-substrate of the SCR 306. In one embodiment, theresistor 110 is fabricated from a silicide-blocked poly-silicon, and isselected with a resistance value (e.g., 1-10 kilo-ohms), which is lowerthan the inherent substrate resistance R_(sub) 341. The resistor 110serves as a shunt for directing small amounts of current to ground 112.Therefore, resistor 110 provides a path for undesirable leakage currentsbetween the trigger device 308 and ground 112, which otherwise mightunintentionally trigger the SCR 302. Furthermore, the resistor 110 willcontrol the so-called trigger and holding currents of the SCR 306.

The triggering device 308 includes a number of serially connected diodesD_(s), (where s is an integer greater than zero) for example, one seriesof PN Junction diode are coupled between the anode 322 and the firstnode 336, which includes the collector of the PNP transistor T2 312 andthe base of the NPN transistor T1 310. The diodes D_(s) are, forexample, three forward biased n-well diodes forming the diode chain 320.An anode of the first diode D₁, for example, PN Junction D₁ in the diodechain 320 is coupled to the pad 104, while the cathode of the last diode(e.g., PN Junction D₃) in the chain 320 is coupled to the first node 336(i.e., trigger gate G1). Each diode D_(s) in the diode chain 320typically has a forward biasing voltage of approximately 0.7 volts.

In operation, the protective SCR circuit 306, which comprises the NPNand PNP transistors T1 310 and T2 312, will not conduct current betweenthe anode 322 and the grounded cathode 112. That is, the SCR 306 isturned off, since there is no high voltage (e.g., ESD voltage) appliedto the pad 104. Rather, only the regular signal or operating voltage ofthe IC appears on the pad 104. In an instance where an ESD event causesan over voltage at the pad 104, the diodes D_(s) in the diode chain 320start to conduct considerable current.

In particular, once a voltage drop of approximately 0.7 volts acrosseach diode in the diode chain 320 occurs, the diodes D_(s) are forwardbiased. Since three diodes are illustratively shown in the diode chain320, a voltage of 2.1 volts must appear across the diode chain 320 toforward bias all three diodes D_(s) in the chain 320.

Initially, a majority of the current flows through the shunt resistor110, since the shunt resistor 110 is in parallel with the substrateresistance R_(sub) 341, which typically has a much greater resistance.However, a portion of the current through the diode chain 320 is fedinto the trigger gate G1 336 of the SCR 306. Once a voltage drop acrossthe shunt resistor 110 (and the parallel intrinsic resistance of thesubstrate R_(sub)) reaches approximately 0.7 volts, the NPN transistorT1 310 is turned on (i.e., triggered). Specifically, the base-emitterdiode, D_(n), for example, a second PN junction diode, of the NPNtransistor T1 310 is forward biased. As such, the NPN transistor T1 310begins to conduct. The collector of the NPN transistor T1 310 providescarriers to the base of the PNP transistor T2 312, which turns on thePNP transistor T2 312. Thus, the DTSCR 302 of FIG. 3 has a turn-onvoltage as between the anode 322 and ground 112 of approximately 2.8volts (2.1V for the diode chain 320 +0.7V for the base-emitter diode).Once both transistors T1 310 and T2 312 of the SCR 306 are turned on,the regenerative conduction process of the SCR 306 enables the ESDcurrent to be quickly shunted to ground 112.

Referring to FIG. 2, curve 218 shows that a voltage of approximately 2.8volts turns on (i.e., triggers) the SCR 306 into a conductive state. TheSCR 306 continues to conduct current at a holding voltage ofapproximately 1.5V and at a clamping voltage in the range of 1.5 to 6volts for higher currents. Thus, the triggering and holding/clampingvoltages for the SCR 306 is less than the 6-9 volt range of the priorart, which may be harmful to the gate oxides of the IC 100.

FIG. 4 depicts a schematic diagram of a second embodiment of the ESDprotection device 402 of the present invention. In particular, FIG. 4depicts a schematic diagram of the DTSCR protection device 402. Theconfiguration of the diode turn-on SCR protection device 402 isconfigured the same as the DTSCR protection device 302 of FIG. 3, exceptthat the SCR is fabricated in a process with an isolated P-well, and thesubstrate resistor 341 is not coupled between the first node 336 andground 112. Furthermore, the poly shunt resistor 110 is not coupledbetween the first node 336 and ground 112. Moreover, one less diode isrequired in the diode chain 320, than used in the diode chain 320 ofFIG. 3. In FIG. 4, the diode chain 320 of the diode turn-on device 408comprises two diodes D_(s).

The SCR 306 of the DTSCR protection device 402 triggers at a lower diodeturn-on voltage than the first embodiment 302 of FIG. 3. Specifically,an ESD event occurring at the pad 104, which is positive with respect toground 112, will forward bias the two diodes D_(s) in the diode chain320 at approximately 1.4 volts. Moreover, once the base to emitterjunction voltage of the NPN transistor T1 310 that forms a base-emitterdiode D_(n) rises to approximately 0.7 volts, the base to emitter diodeD_(n) is forward biased and conducts current, thereby triggering the SCR306. Thus, the SCR 306 of the DTSCR protective device 402 is triggeredat approximately 2.1 volts between the anode 322 and ground 112, ascompared to the 2.8 volts required to trigger the DTSCR protectivedevice 302 of FIG. 3, which has the extra diode in the diode chain 320,and the shunt resistor 110.

FIG. 5 depicts a schematic diagram of a third embodiment of an ESDprotection device 502 of the present invention. In particular, FIG. 5depicts a schematic diagram of the DTSCR protection device 502, suchthat the second node 334 has one or more N+ trigger taps in the N-well,which form trigger gate G2. In this third embodiment, the trigger gateG2 is coupled to the highest available voltage, i.e., the pad 104, via aresistor 504. The pad 104 and resistor 504 ensure a reduction in leakagecurrent by providing a high potential to the N-well of the SCR 306,which turns the PNP transistor T2 312 completely off. Moreover, couplingthe trigger gate G2 to the pad 104 also increases the SCR 306 triggerand holding currents to avoid a latch-up condition. The resistor 504 maybe the intrinsic resistance of the N-well between one or more N+ triggertaps and the base of the PNP transistor 312 of the SCR 306. The resistor504 may alternatively be the resistance of the N-well and/or an externalresistor provided between the terminal of the first node 334 (i.e.,trigger gate G2) and the pad 104. The triggering of this thirdembodiment is similar as described above regarding the DTSCR of FIG. 3.

FIG. 6 depicts a schematic diagram of a fourth embodiment of the ESDprotection device 602 of the present invention. In particular, FIG. 6depicts a schematic diagram of the DTSCR protection device 602, wherethe DTSCR 602 is the same as the DTSCR protection device 302 of FIG. 3,except that the trigger gate G2 at the second node 334 is coupled to apositive supply voltage VDD 604. A large N+ doped region is provided inthe N-well of the SCR 306, adjacent to the P+ doped region formed in theN-well, which serves as the anode 322 of the SCR 306.

The P+ region in the N-well serves dual purposes. First, the P+ toN-well junction forms the emitter-base diode D_(p) of the PNP transistorT2 312. Second, the P+ region and adjacent high doped N+ region alsoform the large emitter-base diode D_(p) in the PNP transistor T2 312,which is connected to the positive supply voltage VDD 604. The couplingof the diode D_(p) to VDD 604 is often needed to cover other ESD stresstypes and polarities. Incorporating the diode D_(p) in the SCR 306avoids the implementation of a more area-consuming separate diode. Thetriggering of this third embodiment is similar as described aboveregarding the DTSCR 302 of FIG. 3. Moreover, similar to the thirdembodiment of FIG. 5, the supply voltage VDD 604 ensures a reduction inleakage current by providing a high potential to the N-well of the SCR306, which turns the PNP transistor T2 312 completely off. Additionally,coupling the trigger gate G2 to the supply voltage VDD 604 alsoincreases the SCR 306 trigger and holding currents to avoid a latch-upcondition.

FIG. 7 depicts a schematic diagram of a fifth embodiment of the ESDprotection device 702 of the present invention. Specifically, FIG. 7depicts a schematic diagram of the DTSCR protection device 702, wherethe DTSCR protection device 702 is the same as the DTSCR protectiondevice 302 of FIG. 3, except that one or more trigger diodes D_(s) arecoupled between the trigger gate G2 of the PNP transistor T2 312 and thetrigger gate G1 of the NPN transistor T1 310.

In particular, two diodes 704 and 706 (i.e., D_(s)) are utilized in thediode chain 320. The diodes 704 and 706 are serially coupled in aforward conductive direction, such that an anode of the first diode 704is coupled to the trigger gate G2 at the second node 334, while thecathode of the second diode 706 is coupled to the trigger gate G1 at thefirst node 336. The placement of the two diodes 704 and 706 of the diodechain 320 allows for a more compact implementation and slightly reducesthe capacitive loading of the pad 104 by a reduced junction capacitance.

During an ESD event at the pad 104, four diodes must be forward biasedto enable the SCR 306 to conduct and serve as a shunt to ground 112.Specifically, the emitter-base junction of the PNP transistor T2 312forms a third diode D_(p) in the diode chain 320, while the base-emitterjunction of the NPN transistor T1 310 forms a fourth diode D_(n) in thediode chain 320. It is noted that the third diode D_(p), formed by theemitter-base junction of the PNP transistor T2 312, is actually thefirst diode in the diode chain 320 from the perspective of the pad 104.Once these four diodes in the diode chain 320 are all forward biased,the SCR 306 triggers, and then shunts the ESD current to ground 112. Itis noted that in this fifth embodiment, the SCR turn-on voltage isapproximately 2.8 volts as between the anode 322 and ground 112.Moreover, the holding voltage of the SCR 306 is approximately 1.5 volts,as shown in FIG. 2. As such, the triggering and holding voltages willproperly protect a gate oxide, as well as other vulnerable semiconductordevices during ESD stress.

FIG. 8 depicts a schematic diagram of a sixth embodiment of the ESDprotection device 802 of the present invention. In particular, FIG. 8depicts a schematic diagram of a diode turn-on NMOS (DTNMOS) protectiondevice 802 of the present invention. The configuration of the diodeturn-on DTNMOS protection device 802 in this sixth embodiment is thesimilar as the DTSCR protection device 302 of FIG. 3, except that anNMOS device 804 is used instead of the SCR 306.

In particular, the serially connected turn-on diodes 320 are coupledbetween the pad 104 and a gate of the NMOS device 804 in the forwardbias direction. More specifically, the anode of a first diode 812 in thediode chain 320 is coupled to the pad 104, while the cathode of the lastdiode 814 in the diode chain 320 is coupled to the gate of the NMOSdevice 804. Each diode is formed in a separate N-well, thereby allowingpotential isolation from the common P-substrate. The diodes D_(s) in thediode chain 320 may be sized to accommodate low current flow, which hasa maximum current of approximately 10 nanoamps at the nominal voltage atthe pad 104, as well as over the entire operating temperature range ofthe IC 100.

One end of a shunt resistor 110 is also coupled to the gate of the NMOSdevice 804. As such, the gate of the NMOS device, the last diode 814 inthe diode chain 320, and the shunt resistor 110 define first node 810.The other end of the shunt resistor 110 is coupled to ground 112. Theshunt resistor 110 has a resistance in the range of 1-10 Kohms. In theexemplary embodiment, three diodes D_(s) are depicted in the diode chain320. However, the number of diodes D_(s) may be varied, as long as undernormal circuit conditions, the maximum voltage at the pad 104 does notcause any considerable current leakage (e.g., above 100 nanoamps) toground 112 via the diode chain 320 and the shunt resistor 110.Typically, the overall number of diodes D_(s) in the diode chain 320should not exceed 4 or 5 diodes. The typical voltage drop during normaloperating conditions across each diode in the diode chain 320 is between0.3 to 0.4 volts in order to keep the leakage current sufficiently low.During an ESD event, the voltage drop across each diode in the diodechain 320 is typically 0.7 volt.

The drain of the NMOS device 804 is coupled to the pad 104, while thesource of the NMOS device 804 is coupled to ground 112. A parasitic NPNtransistor 806, which is inherent to the NMOS device 804, is also shownin FIG. 8. In particular, the N+ doped regions forming the drain andsource of the NMOS device 804 also respectively form the collector andemitter of the parasitic NPN transistor 806, while the P-substrate formsthe base of the parasitic NPN transistor 806.

The NMOS device 804 is turned on by an ESD event occurring at the pad104, such that a voltage drop of approximately 0.7 volts forms acrosseach diode in the diode chain 320. Once the diodes D_(s) in the diodechain 320 are forward biased, the diodes D_(s) conduct and the currentflows through the shunt resistor 110. When the voltage across the shuntresistor 110 rises above the gate threshold voltage (e.g., 0.5 volts) ofthe NMOS device 804, the NMOS device 804 turns on, thereby allowing thecurrent to shunt to ground 112. Specifically, the current flows from thedrain, and through the source of the NMOS device 804 to ground 112.Moreover, the parasitic NPN transistor 806 will conduct current throughits collector and emitter to ground 112. As such, the NMOS device 804(along with the parasitic NPN transistor 806) shunts the current fromthe pad 104 to ground 112. It is noted that the gate biasing of the NMOSdevice 804 by the diode chain 320 helps reduce the trigger voltage ofthe parasitic NPN transistor 806, as well as providing uniformtriggering where multiple NMOS fingers are present.

An optional limiter diode 808 may also be coupled to the first node 810and ground 112. In particular, the limiter diode 808 is coupled in aforward conducting direction from the gate of the NMOS device 804 toground 112. The limiter diode 808 ensures that the voltage at the gatedoes not exceed a potential that may cause hot carrier damage to thegate oxide, in conjunction with the high currents flowing in the MOSdevices under ESD operation. In particular, the limiter diode may have aforward biasing voltage of approximately 0.7 volts, which is above thegate threshold voltage of 0.5 volts.

FIG. 9 depicts a schematic diagram of a seventh embodiment of the ESDprotection device 902 of the present invention. In particular, FIG. 9depicts a schematic diagram of the diode turn-on NMOS (DTNMOS)protection device 902, where the diode turn-on DTNMOS protection device902 is similar as the DTNMOS protection device 802 of FIG. 8. However,the parasitic NPN transistor 806 is used as the triggering point toturn-on the NMOS device 804, rather than the gate of the NMOS device804.

In particular, the gate of the NMOS device 804 is coupled to ground 112to turn off any MOS current. Further, the diode chain 320 is coupled tothe base of the parasitic NPN transistor 806, which is also coupled toground 112 via the shunt resistor 110. The intrinsic resistance R_(sub)341 of the substrate is also shown as coupled to ground 112 in parallelwith the shunt resistor 110.

During an ESD event at the pad 104, the diodes D_(s) in the diode chain320 conduct, and the current flows through the shunt resistor 110. Thediodes D_(s) in the diode chain 320 are forward biased at approximately0.7 volts each. When the voltage across the shunt resistor 110 risesabove the base-emitter forward biasing voltage (e.g., 0.7 volts) of theparasitic NPN transistor 806, the parasitic NPN transistor 806 turns on(i.e., conducts), thereby allowing the current to flow from thecollector, through the emitter, to ground 112. As such, the NMOS device804 (along with the parasitic NPN transistor 806) is utilized to shuntthe current from the pad 104 to ground 112 at a triggering voltage ofapproximately 2.8 volts and at a holding voltage of approximately 5volts.

FIGS. 10-12 depict various complementary ESD protection deviceembodiments of the present invention. For each of these embodiments, thetrigger device 308 is coupled between a trigger gate G2 334 of the PNPtransistor T2 312 of the SCR 306 and ground 112, instead of between thepad 104 and the trigger gate G1 336 of the NPN transistor T2 310 of theSCR 306.

In particular, FIG. 10 depicts a schematic diagram of a DTSCR protectiondevice 1002, which comprises the SCR 306 and the triggering device 308.The SCR 306 is the same as described in the other embodiments above,having first and second trigger gates G1 and G2. It is noted that then-well is floating, such that there is no intrinsic n-well resistanceR_(nwell). It is also noted that a shunt resistor 110 is not utilized,as discussed with regard to the embodiment 402 of FIG. 4.

The triggering device 308 comprises the diode chain 320 formed by theserially connected diodes D_(s), which are coupled between the triggergate G2 at the second node 334 and ground 112. As such, this eighthembodiment 1002 may be considered as complementary to the secondembodiment 402 of FIG. 4, which has the trigger device 308 coupledbetween the pad 104 and the trigger gate G1 of the NPN transistor T1 310at the first node 336.

The diode chain 320 illustratively comprises two diodes D_(s), which arein the forward bias direction from the trigger gate G2 334 to ground112. When an ESD event occurs at the pad 104, the emitter-base junctionof the PNP transistor T2 312 acts as a diode D_(p), and begins toconduct. The diodes D_(s) in the diode chain 320 also begin to conductand the current flows to ground 112. Once the voltage potential acrossthe emitter-base diode D_(p) of the PNP transistor T2 312 and each diodeD_(s) in the diode chain 320 rises to approximately 0.7 volts, theemitter-base diode D_(p) of the PNP transistor T2 312 and diodes D_(s)in the diode chain 320 are all forward biased. The current flows fromthe emitter to the collector (which also forms the base of the NPNtransistor T1 310) of the PNP transistor T2 312, to initiate theregenerative conduction process of the SCR 306.

The voltage potential occurring across the diode chain 320 (e.g., havingtwo diodes D_(s) between the trigger gate G2 and ground) isapproximately 1.4 volts, while the voltage drop across the emitter-baseof the PNP transistor T2 312 is approximately 0.7 volts. Thus, the PNPtransistor T2 312 of the SCR 306 will trigger when the emitter-basediode D_(p) of the PNP transistor T2 312 and diode chain 320 reachesapproximately 2.1 volts. Referring to FIG. 2, both the triggeringvoltage and the holding voltage are below the voltage region 210 (i.e.,less than 6 volts), which may be considered harmful (e.g., destructive)to the gate oxides.

FIG. 11 depicts a schematic diagram of a ninth embodiment of the ESDprotection device 1102 of the present invention. In particular, FIG. 11depicts a schematic diagram of the DTSCR protection device 1102, wherethe DTSCR protection device 1102 is the same as the DTSCR protectiondevice 1002 of FIG. 10, except that a resistor 504 is coupled betweenthe pad 104 and the trigger gate G2 of the PNP transistor T2 312, at thesecond node 334. The resistor 504 is, illustratively, the intrinsicresistance of the n-well, as discussed with regard to FIG. 5.Specifically, the trigger gate G2 is coupled to the highest availablevoltage, i.e., the pad 104, via a resistor 504. The pad 104 and resistor504 ensure a reduction in leakage current by providing a high potentialto the N-well of the SCR 306, which turns the PNP transistor T2 312completely off.

The trigger device 308 illustratively comprises three diodes D_(s), forexample, one series of PN junction diode in the diode chain 320. When anESD event occurs at the pad 104, the emitter-base junction of the PNPtransistor T2 312 acts as a diode D_(p), for example a second PNjunction diode, and is forward biased at approximately 0.7 volts. Thediodes D_(s) in the diode chain 320 also begin to conduct. Once thevoltage potential across each diode D_(s) in the diode chain 320 risesto approximately 0.7 volts, the diodes D_(s) in the diode chain 320 arealso forward biased. As such, the voltage drop occurring across thediode chain 320 is approximately 2.1 volts. Thus, the PNP transistor T2312 of the SCR 306 will trigger once the voltage between the anode 322and ground 112 reaches approximately 2.8 volts. Referring to FIG. 2,both the triggering voltage and the holding voltage are below (i.e.,less than 6 volts) the voltage region 210, which is considered harmfulto the gate oxides.

FIG. 12 depicts a schematic diagram of a tenth embodiment of the ESDprotection device 1202 of the present invention. In particular, FIG. 12depicts a schematic diagram of the DTSCR protection device 1202, wherethe DTSCR protection device 1202 is the same as the DTSCR protectiondevice 1102 of FIG. 11, except that the shunt resistor 110 is coupledbetween the pad 104 and the trigger gate G2 of the PNP transistor T2312, at the second node 334. Similar to the embodiment of FIG. 3, theshunt resistor 110 is parallel with the resistor 504 and has aresistance value much lower than the intrinsic resistance 504. As such,the current produced by an ESD event at the pad 104 flows initiallythrough the shunt resistor 110, rather than the resistor 504,illustratively the intrinsic resistance 504 of the n-well. The shuntresistor 110 provides a path for undesirable leakage currents betweenthe trigger device 308 and ground 112, which otherwise mightunintentionally trigger the SCR 306. Furthermore, the shunt resistor 110will control the so-called trigger and holding currents of the SCR 306.

FIGS. 13-16 depict schematic diagrams of various SCR protection devicesutilizing one or more coupling capacitors in the ESD protectivecircuitry. FIG. 13 depicts a schematic diagram of the DTSCR protectiondevice 1302, where the DTSCR protection device 1302 is the same as theDTSCR protection device 1002 of FIG. 10, but includes capacitivegrounding via a coupling capacitor 1304. In particular, the couplingcapacitor 1304 is coupled in series between the diode chain 320 andground 112. During a transient ESD event, the transient current willflow through the coupling capacitor 1304, while any non-transient (DC)current will be blocked by the coupling capacitor 1304. The couplingcapacitor 1304 may have a capacitance value in the range of 1 pF(pico-Farads to 1 nF (nano-Farads). Once the emitter-base diode D_(p) ofthe PNP transistor T2 312, as well as the diodes D_(s) in the diodechain 320 are forward biased (e.g., 0.7 volts), the SCR 306 turns on andshunts the ESD current from the pad 104 to ground 112. FIG. 13 a depictsa schematic diagram of the DTSCR protection device 1303, where the DTSCRprotection device 1303 is the same as the DTSCR protection device 1002of FIG. 10, but includes capacitive coupling via a coupling capacitor1304. In particular, the coupling capacitor 1304 is coupled in seriesbetween the diode chain 320 and the pad 104. During a transient ESDevent, the transient current will flow through the coupling capacitor1304, while any non-transient (DC) current will be blocked by thecoupling capacitor 1304. The coupling capacitor 1304 may have acapacitance value in the range of 1 pF(pico-Farads to 1 nF(nano-Farads). Once the emitter-base diode D_(p) of the PNP transistorT1 310, as well as the diodes D_(s) in the diode chain 320 are forwardbiased (e.g., 0.7 volts), the SCR 306 turns on and shunts the ESDcurrent from the pad 104 to ground 112.

FIG. 14 depicts a schematic diagram of the SCR protection device 1402,which is the configured as the SCR protection device 1302 of FIG. 13,except that the turn-on diodes D_(s) in the diode chain 320 are notutilized. That is, the coupling capacitor 1304 is used instead of theturn-on diodes D_(s) in the diode chain 320, such that the protectiondevice 1302 may be said to be a capacitive turn-on SCR (CTSCR). Inparticular, the SCR 306 is turned on by capacitively grounding acoupling capacitor 1304 directly between the second gate G2 334 of theSCR 306 and ground 112. During a transient ESD event, the transient ESDcurrent will initially flow through the coupling capacitor 1304, whileany non-transient (DC) current will be blocked by the coupling capacitor1304.

Moreover, in the initial phase of the ESD pulse, the coupling capacitor1304 pulls the trigger gate G2 334 of the SCR 306 to approximatelyground potential. In other words, the voltage drop across the capacitoris practically negligible. Once the emitter-base diode D_(p) of the PNPtransistor T2 312 is forward biased at approximately 0.7 volts, the SCR306 turns on and shunts the ESD current from the pad 104 to ground 112.As such, the SCR 306 turn-on voltage across the emitter-base diode D_(p)of the PNP transistor T2 312 and the coupling capacitor 1304 isapproximately 0.7 volts, which is below the voltage region 210 (i.e.,less than 6 volts) that is considered harmful to the gate oxides.

FIG. 15 depicts a schematic diagram of the SCR protection device 1502,which is configured as the SCR protection device 1402 of FIG. 14, exceptthat the coupling capacitor 1504 is coupled between the pad 104 and thetrigger gate G1 336 of the NPN transistor T1 310. During an ESD event,initially the transient current will flow through the coupling capacitor1504, while any non-transient (DC) current will be blocked by thecoupling capacitor 1504. Once the base-emitter diode D_(n) of the NPNtransistor T1 310 is forward biased (e.g., 0.7 volts), the SCR 306 turnson and shunts the ESD current from the pad 104 to ground 112. As such,the SCR 306 turn-on voltage across the base-emitter diode D_(n) of theNPN transistor T1 302 and the coupling capacitor 1304 is approximately0.7 volts (i.e., less than 6 volts), which is below the voltage region210 that may be harmful to the gate oxides.

FIG. 16 depicts a schematic diagram of the SCR protection device 1602,which is the configured as a combination of the SCR protection devices1402 and 1502 of FIGS. 14 and 15. In particular, capacitive grounding isprovided by connecting coupling capacitor 1304 directly between thesecond gate G2 334 of the SCR 306 and ground 112. Further, couplingcapacitor 1504 is coupled between the pad 104 and the trigger gate G1336 of the NPN transistor T1 310. During an ESD event, initially thetransient currents will flow through the coupling capacitors 1304 and1504, while any non-transient (DC) current will be blocked by thecoupling capacitors 1304 and 1504. Once the emitter-base diode D_(p) ofthe PNP transistor T2 312, or the base-emitter diode D_(n) of the NPNtransistor T1 310 is forward biased (e.g., 0.7 volts), the SCR 306 turnson and shunts the ESD current from the pad 104 to ground 112.

It is noted that the coupling capacitor 1304 coupled to the second gate334, as shown and discussed with respect to FIGS. 13, 14, and 16, may beformed by the intrinsic capacitance formed between the N-well and thesubstrate. Alternatively, the capacitor 1304 may be formed by anexternal on-chip capacitor.

It is also noted that the coupling capacitor 1504 coupled to the firstgate 336, as shown and discussed with respect to FIGS. 15 and 16, isformed by an external on-chip capacitor. The coupling capacitor 1504 hasa similar capacitive value range as the coupling capacitor 1304 (i.e., acapacitance value in the range of approximately 1 pico-Farad (pF) to 1nano-Farad (nF)).

In another embodiment, the coupling capacitor 1304 may be used toillustrate an intrinsic capacitance formed between two supply lines,such as VDD and ground. Referring to FIG. 14, the N-well at the secondgate G2 334 is coupled to the supply line VDD via phantom line 1404.When the second gate G2 334 is illustratively tied to VDD, the intrinsiccapacitance formed between VDD and ground 112 is utilized. The intrinsiccapacitance may have a capacitance value in the range of approximately 1pF to 1 nF. In this latter embodiment, the coupling capacitor 1304 nowrepresents the capacitance between two supply lines (e.g., VDD andground), as opposed to representing the intrinsic capacitance betweenthe N-well and ground, as discussed above. Furthermore, although notshown, additional external on-chip capacitors may be added either inparallel to the intrinsic capacitance to increase the overallcapacitance therebetween, or serially to reduce the intrinsiccapacitance between the supply lines. The advantages of utilizing theintrinsic capacitance between the supply lines is because this intrinsiccapacitance is usually very large it does not require any extra area forimplementation.

In yet another embodiment, an external on-chip resistor R 1406 may becoupled between the N-well of the second gate G2 334 and VDD. Theexternal resistor 1406 may have a resistive value in a range ofapproximately 1 ohm to 10 kohms. The external resistor 1406 is utilizedto limit the current through the capacitor to improve the triggering ofthe SCR.

FIGS. 17 and 18 depict schematic diagrams of the SCR protection devicesutilizing a number of serially coupled MOS devices as the turn-on device308 of the ESD protective circuitry, rather than the diode chain 320.The ESD protection devices 1702 and 1802 of FIGS. 17 and 18 are thesimilar to the embodiment shown in FIG. 3, except a plurality ofserially connected MOS devices are coupled between the pad 104 and thetrigger gate G1 336 of the NPN transistor T1 310 of the SCR 306.

In particular, FIG. 17 illustratively depicts three NMOS devices 1708serially coupled between the pad 104 and the trigger gate G1 336 of theNPN transistor T1 310. Alternately, FIG. 18 illustratively depicts threePMOS devices 1808 serially coupled between the pad 104 and the triggergate G1 336 of the NPN transistor T1 310. As similarly discussed abovewith regard to the diode turn-on devices D_(s) of FIGS. 3-16, the numberof MOS devices (i.e., NMOS or PMOS) that are serially connected may varybetween 1 to 8 devices.

Referring to FIG. 17, the gate and the drain of the NMOS devices 1708are coupled to a high potential, compared to the source(“Diode-connected MOS”). As such, the NMOS devices 1704 are normally inan “on” state. Once a threshold voltage (i.e., “knee voltage” as similarto a forward biased diode) (e.g., 0.2-0.7 volts) is exceeded, thecurrent rapidly increases such that the NMOS devices 1708 act as forwardbiased diodes.

During an ESD event occurring at the pad 104, current initially flowsthrough the NMOS devices 1704 to ground 112, via the shunt resistor 110.Once the voltage potential across each NMOS device 1704 exceeds thethreshold voltage, the current through the shunt resistor 110 increases,thereby increasing the voltage across the shunt resistor 110. When thevoltage across the shunt resistor 110 reaches approximately 0.7 volts,the base-emitter diode of the NPN transistor T1 310 is forward biased,thereby triggering the SCR 306.

In an instance where three NMOS devices 1704 are used having a thresholdvoltage of approximately 0.5 volts each, the voltage potential acrossthe three NMOS devices 1704 is approximately 1.5 volts. As such, the SCR306 turn-on voltage across the base-emitter diode D_(n) (0.7 volts) ofthe NPN transistor T1 302 and the NMOS devices 1704 is approximately 2.2volts, which is below the voltage region 210, (i.e., less than 6 volts)that may be harmful to the gate oxides.

Referring to FIG. 18, the gate and drain of each PMOS devise 1808 iscoupled to a low voltage potential (e.g., VDD 1804) compared to thesource. As such, the PMOS devices 1804 are normally in an “on” state.During an ESD event, the same analysis may be applied to the PMOS device1804 of FIG. 18 as applied to the NMOS devices 1704 of FIG. 17.

FIG. 19 depicts a schematic diagram of the SCR protection device 1902having a reversed biased Zener diode 1908 as the turn-on device 308 ofthe ESD protective circuitry 1908, rather than the diode chain 320 ofFIGS. 3-16. The ESD protection device 1902 is the same as the embodimentof FIG. 3, except that a reversed biased Zener diode 1908 is coupledbetween the pad 104 and the trigger gate G1 336 of the NPN transistor T1310 of the SCR 306. During an ESD event occurring at the pad 104,current flows from the anode 322 to ground 112, via Zener diode 1908 andthe shunt resistor 110.

Once the voltage across the Zener diode 1908 reaches the breakdownvoltage (e.g., 3-6 volts), the current through the shunt resistor 110increases, thereby increasing the voltage potential across the shuntresistor 110. When the voltage across the shunt resistor 110 reachesapproximately 0.7 volts, the base-emitter diode D_(n) of the NPNtransistor T1 310 is forward biased, thereby triggering the SCR 306 intoconduction, which shunts the ESD current from the pad 104 to ground 112.

Zener diodes are usually formed by a junction, such as a P-type lightlydoped drain (PLDD) doping) and a N-type highly doped region (N+), or aN-type lightly doped drain (NLDD) doping and a P-type highly dopedregion (P+), or a combination of both PLDD and NLDD doping. However,these Zener diodes have breakdown voltages of typically 6-12V, which istoo high for the protection of may ultra thin gate oxides.

FIG. 31 depicts a cross-sectional view of a Zener diode triggeringdevice 1908 of the present invention. In particular, an N-well 3104formed on a P-substrate (not shown) comprises a P+ doped region 3106formed adjacent to an N+ doped region 3108, which forms a junction 3112therebetween. A portion of the P+ doped region has a silicide layer3110, where a contact is provided to form the anode 322 of the Zenerdiode 1908. Likewise, a portion of the N+ doped region 3108 has asilicide layer 3110, where a contact is provided to form the cathode ofthe Zener diode 1908. An area between the silicided layers 3110 and overthe junction 3112 is silicide blocked to prevent a surface shortcircuit. In one embodiment, the N+ to P+ junction 3112 establishes abreakdown voltage of typically 3-6V.

One skilled in the art will recognize that attentive process evaluationmust be performed to determine any increased leakage current in such astructure, which may have a detrimental impact on the application in anESD protection device. In worst case, the SCR 306 turn-on voltage acrossthe base-emitter diode D_(n) of the NPN transistor T1 302 and the Zenerdiode 1908 is approximately 6.7 volts, which is in the low end of thevoltage region 210 (i.e., approximately 6 volts), which may be harmfulto the gate oxides.

FIG. 20 depicts a schematic diagram of an ESD protection device 2002 foran integrated circuit (IC) 100 having a plurality of various (“mixed”)supply voltages 2004 ₁ through 2004 _(n) (collectively, mixed supplyvoltages 2004). The embodiment utilizes the capacitive coupling toground of supply lines other than a protected supply line. Theembodiment of FIG. 20 protects the IC circuitry from undesirable ESDdischarge occurring at one of the supply voltage lines 2004. The ESDprotection device 2002 comprises the capacitance turn-on SCR (CTSCR)1402, as discussed above with regard to FIG. 14, as well as the diodeturn-on SCR (DTSCR) device 1002, as discussed above with regard to FIG.10.

The supply voltage lines 2004 have parasitic capacitance 2006 (e.g.,parasitic capacitance 2006 ₁ through 2006 _(m)) occurring between eachsupply voltage line and ground 112. That is, the supply voltage lines2004 (and all devices connected to the supply line 2004) act asdistributed plates, such that parasitic capacitance 2006 is generatedbetween the supply lines 2004 and ground 112. The parasitic capacitance2006 may be used to trigger the SCR 306 instead of the couplingcapacitor 1304 discussed in FIGS. 13-15.

Referring to FIG. 20, the protective circuitry 2020 is coupled betweentwo supply voltage lines 2004 and ground 112. The anode 322 of the SCR306 is coupled to a different voltage supply line (e.g., supply voltages2004 ₁), than the supply voltage line (e.g., supply voltages 2004 ₂)coupled to the trigger gate G2 334 of the SCR 306. The protectivecircuitry 2020 may be utilized to protect the supply line 2004 ₁ versusground 112. Although the latter supply line 2004 ₁ is illustrativelyconsidered as being subjected to the ESD stress, it is noted that theother potential supply lines are protected as well, but are not beingconsidered as operating under ESD stress conditions. That is, the anode322 may be coupled to a supply line having a potential that is the samepotential (but different supply domain), a lower potential, or a higherpotential than the potential of the gate G2 334.

In particular, the trigger gate G2 334 of the PNP transistor 312 of theSCR 306 is coupled to the “lower potential” supply voltage line 2004 ₂,which illustratively has a potential of +2.5 volts. The gate G2 334 ofthe SCR 306 is coupled to supply voltage line 2004 ₂ via the seriallyconnected trigger diodes 2010 from the gate G2 334 to the supply voltage2004 ₂.

The emitter of the PNP transistor 312, which forms the anode 322 of theSCR 306, is coupled to supply voltage line 2004 ₁ via the seriallyconnected diodes 2008. The holding voltage diodes 2008 (e.g., 3 diodes)are used to maintain the holding voltage of the SCR 306 above the higherpotential supply voltage 2004 ₁ (e.g., 3.3 volts) to eliminate the riskof latch-up. The supply voltage line 2004 ₂ is then coupled to ground112 (i.e., a reference voltage supply line Vss 2004 _(n+1)) via theparasitic capacitance 2006 ₁, which exists between the voltage supplylines 2004 ₂ and 2004 _(n+1) (i.e., ground 112). The first trigger gateG1 336 of the NPN transistor 310 is coupled to ground 112 via theintrinsic substrate resistance 341 of the SCR 306. Additionally, theemitter of the NPN transistor 310 is also coupled to ground 112 to formthe cathode of the SCR 306.

The embodiment of FIG. 20 must operate under three conditions. A firstcondition is during power-up of the mixed voltage IC 100, where thesupply voltage lines 2004 are turned-on in an arbitrarily sequence. Asecond condition is under normal operation, where the SCR 306 must notinterfere with normal operation of the IC. That is, a latch-up conditionmust be prevented. The third condition is under an ESD stress condition,where the IC is not powered up with DC supplies, and the SCR's 306 mustquickly shunt the ESD current to ground 112.

Each of these three conditions may be fulfilled by providing an adequatenumber of diodes 2008 and 2010 in the anode 322 and gate G2 334 paths.It is noted that the holding diodes 2008 in the anode path 322 areprovided to increase the holding voltage in the SCR 306 conductive “on”state, at a voltage above the supply voltage to prevent a latch-upcondition. As discussed above regarding the desired holding voltage ofthe SCR 306, a person skilled in the art will easily determine thenumber of holding diodes required in the ESD protection circuit 2002.The holding diodes 2008 are positioned in the ESD discharge path, andmust be sufficiently large to withstand the same amount of stresscurrent as the SCR 306.

The trigger diodes 2010 at the trigger gate G2 341 are optionallyprovided to fulfill the conditions given by power-up constraints andlatch-up prevention. The trigger diodes 2010 may be minimal in size,since only small amounts of trigger currents (as compared to an ESDstress current) are conducted by the SCR 306.

The power-up condition dictates the number of holding and triggeringdiodes 2008 and 2010 that are utilized. In the worst case during powerup, where the supply line connected to the anode 322 is turned on first,while the supply line coupled to the gate G2 334 is still effectivelycoupled to ground 112, the SCR 306 must not be triggered. Under thisworst-case condition, the diode chain consisting of the holding diodes2008, the internal emitter-base diode of the PNP transistor 312, and thetrigger diodes 2010 are forward biased.

To avoid SCR triggering during power-up, the sum of the diode voltagesacross this entire diode chain (i.e., holding diodes 2008, emitter-basediode D_(p), and trigger diodes 2010) must at least compensate for theapplied anode supply voltage. For example, where the anode 322 iscoupled to the 3.3 volt supply line 2004 ₁, a total of seven diodes mustbe utilized in the protective circuit 2020. That is, three holdingdiodes 2008, the emitter-base diode D_(p) of the PNP transistor 312, andthe three trigger diodes 2010 are required.

Under the non-powered ESD stress condition, all of the voltage supplylines 2004 ₁-2004 _(n) are capacitively coupled to ground, due to theparasitic connection 2006 between each line 2004 and ground 112. When apositive ESD event occurs at one of the protected supply lines (e.g.,2004 ₁ through 2004 _(n)), the SCR 306 turns on once the voltage at theprotected supply line exceeds the aggregate voltage across the holdingdiodes 2008, the emitter-base diode D_(p) of the PNP transistor 312, andthe trigger diodes 2010.

It is noted that typically, the maximum number of series diodes in theDTSCR protection device should not exceed 4-5 diodes for limiting theleakage current. However, the present embodiment allows the use of theDTSCR protection device 2002 for higher voltages, since a greater numberof turn-on diodes are provided. Further, during normal circuitconditions, the voltage drop across each diode is reduced from of theapplied supply voltages biasing the diodes.

It is also noted that the complementary DTSCRS may also be used toprotect the supply lines 2004, rather than simply being limited to theprotection of an I/O, as illustratively shown in FIGS. 10-13. Inparticular, one or both branches of the complementary SCR may be used toprotect the supply lines with the same or lower voltage level than theG2 reference potential. Such supply line protection may be used inapplications where there is no power-up sequence, such that all thesupply lines 2004 are ramped up simultaneously.

FIG. 21 depicts a schematic block diagram representing an ESD protectioncircuit 2102 having reduced parasitic capacitance. In particular, thecapacitance reduction embodiment of FIG. 21 comprises the ESD protectiondevice 102 (e.g., DTSCR or NMOS devices of FIGS. 3-19) coupled betweenthe pad 104 and ground 112, as discussed above. The parasiticcapacitance 2006 (Cesd) is shown existing between the anode 322 of theESD protection device 106 and ground 112. The parasitic capacitance 2006has a capacitance in the range of typically 200 to 3000 femto-Farads.This parasitic capacitance increases with the size of the ESD protectiondevice 106 included on the input pad 104, while a larger size of the ESDprotection device provides a higher protection level. Although theembodiment is discussed in terms of the input pad 104, one skilled inthe art will understand that the same principles apply to an output orbi-directional pad.

A capacitance reducing diode 2104 is serially coupled in the forwardconductive direction between the protective input pad 104 and the anode322 of the ESD protection device 106. The diode 2104 adds a smallvoltage drop once the protective circuit 2102 is in the ESD mode ofoperation. The diode 2104 is typically implemented in a well (e.g.,N-well) to isolate it from the substrate. The diode 2104 has a smallparasitic junction capacitance value (e.g., 30 to 100 fF), which is muchsmaller in value than the parasitic capacitance Cesd 2006 of the ESDprotection device 106. The diode parasitic capacitance Cdio 2106 and theESD protection device capacitance Cesd 2006 are coupled in seriesbetween the pad 104 and ground 112. The overall capacitance C_(t) of theprotection device 2102 is reduced by the serial relationship (i.e.,C_(t)=(Cdio*Cesd)/(Cdio+Cesd)) of the parasitic capacitance. The signalpresent at the pad 104 will only be influenced by the overallcapacitance C_(t).

Further reduction in the parasitic capacitance of the ESD protectioncircuit 2102 may be provided by coupling the anode of the ESD protectiondevice 106 to a (positive) supply voltage line 2004, via resistor 2108(e.g., 1K to 100K Ohms. Under normal circuit operation, the diode 2104becomes reversed biased, which further reduces the parasitic capacitanceCdio 2106 of the diode 2104. The further reduction in the parasiticcapacitance Cdio 2106 of the diode is due to the non-linear dependencybetween junction capacitance and reverse biasing. During an ESD event,current through the resistor 2108 is limited to a negligible amount. Assuch, the diode 2104 is forward biased and the ESD protection device 106may quickly shunt the transient ESD current to ground 112, as discussedabove.

In one embodiment, the ESD protection circuit 2102 is used forhigh-speed circuits. In order to increase the speed of the circuit 100,the parasitic capacitances that load an input signal must be very small.As such, the ESD protection circuit 2102 must not add more thantypically 50 to 200 femto-Farads (fF) of parasitic capacitance.

FIGS. 22-24 depict schematic diagrams of various embodimentsincorporating the teachings of the generic embodiment 2102 of FIG. 21.FIG. 22 depicts a schematic diagram of an ESD protective circuit 2202having the capacitance reducing diode 2104 coupled to the DTSCR 302 ofFIG. 3. Moreover, the first diode in the diode chain 320 of the triggerdevice 308 is used as the capacitance reducing diode 2104. The voltagesupply line (VDD) 2004 is coupled to the cathode of the capacitancereducing diode 2104 in the diode chain 320 via resistor 2108. As such,the overall capacitance C_(t) of the protection device 2102 is reducedby the serial relationship between the parasitic capacitance of thecapacitor reducing diode 2104 and the parasitic capacitances from othertrigger diodes in the trigger device 308 to ground 112, as discussedabove.

FIG. 23 depicts a schematic diagram of an ESD protective circuit 2302having the capacitance reducing diode 2104 coupled to a SCR 306, wherethe capacitance reducing diode 2104 may already be present in the formof the upper diode of the holding voltage diodes. The capacitancereducing diode 2104 may also be used for other types of ESD protectiondevices. FIG. 24 illustratively depicts a schematic diagram of an ESDprotective circuit 2402 having the capacitance reducing diode 2104coupled to a grounded-gate NMOS ESD protection device 2406. It should beunderstood from the teachings in the embodiments of FIGS. 21-24, thatthe capacitive reducing diode 2104 may be used with at least any of theembodiments depicted in FIGS. 3-19 above. Alternately, the capacitivereducing diode 2104 may be used with other triggering devices, such as agrounded-gate SCR (GGSCR).

FIG. 25 depicts a schematic diagram of the ESD protection circuit 302having SCR turn-on diodes act as a Darlington transistor pump 2502. TheESD protection circuit comprises the SCR 306 coupled between the pad 104and ground 112. The diode turn-on device 308 is illustrativelyrepresented by a three stage Darlington transistor 2502, where eachstage 2512 ₁ to 2512 ₃ (collectively stages 2512) corresponds to a diodeD_(s) in the serially coupled diode chain 320. Specifically, the diodesD_(s) in the chain 320 of the DTSCR 302 form parasitic PNP transistorswith the P-substrate (not shown). That is, the P-substrate forms thecollectors of each stage 2512, which is normally coupled to ground 112.The collector of each stage 2512 carries part of the current from eachdiode (i.e., transistor stage 2512) to the grounded P-substrate (notshown) of the IC 100, thereby increasing the leakage current to thesubstrate during normal operation and the likelihood that the SCR 306will fail to trigger.

To alleviate this current loss problem, a plurality of P+ ties 2520 maybe formed in the P-substrate and close to the N-well diodes, therebycoupling the collectors of the Darlington transistor 2502 better to atrigger gate, such as trigger gate G1 336, as shown in FIG. 25. The P+ties may be also used in instances where the diodes D_(s) are formed ina P-well that is isolated from the P-substrate such as available in amanufacturing process for the IC 100 with “isolated P-well”/“DeepN-well”. Furthermore, a manufacturing process for the IC 100 with“triple-wells” (a first N-well inside a quasi-deep P-well, inside a deepN-well) utilizes the Darlington effect that also collects all currentswithout loss. As such with the aforementioned techniques, the collectorcurrents I_(c) from each stage 2512, as well as the base current I_(b)of the last stage 2512 ₃ of the Darlington transistor 2502 are coupledto the trigger gate (e.g., trigger gate G1).

Although FIG. 25 depicts the Darlington pump 2502 coupled to the triggergate G1 336 of the SCR 306, it is understood that the Darlington pump2502 is alternately coupled to the trigger gate G2 334 for thosecomplementary embodiments having the diode chain 320 coupled to thetrigger gate G2 334 of the SCR 306.

It is noted that in an embodiment where a complementary DTCR is used,such as those embodiments depicted in FIGS. 10-13, the Darlingtongenerated substrate current is not lost in the P-substrate as discussedabove. Referring to FIG. 11, the diode chain 320 is coupled to thesecond gate G2 334 formed at the PNP transistor T2 312. As such, thecollectors of each Darlington stage, as well as the base of the laststage, are inherently coupled to the P-substrate. The trigger currenteffective at the second gate G2 334 equals, in this case, the sum of thecollector (substrate) currents I_(c1-3) and the base current I_(b3) ofthe last stage of the Darlington chain.

FIG. 32 depicts a schematic diagram of the ESD protection circuit 1102having a complementary SCR turn-on Darlington transistor pump 3202. Infact, FIG. 32 corresponds to the schematic drawing of FIG. 11. Thetrigger current at the trigger gate G2 334 equals the sum of thecollector currents (I_(c1)+I_(c2)+I_(c3)) of each Darlington stage 2512,plus the base current I_(b3) of the last Darlington stage (e.g., 2512₃). Therefore, the current that is lost due to the Darlington effect,which results from serially coupling triggering diodes D_(s) to the gateG1 336 of the NPN transistor T1 310, is automatically recovered and usedfor triggering the gate G2 334 in the complementary DTSCR embodiments.

FIG. 26 depicts a schematic diagram of a temperature compensated triggerdevice 2608 of the ESD protection circuit 102. The purpose of thetemperature compensating triggering device 2608 is to allow the leakageand triggering currents to remain within a particular operating range,regardless of the operating temperatures. That is, the triggering pointand leakage currents are substantially independent of the operatingtemperatures of the IC 100.

The temperature compensated trigger device 2608 comprises at least oneMOS device, such as a PMOS device 2610 serially coupled to an NMOSdevice 2612, which is serially coupled to a diode chain 320. Inparticular, the source of the PMOS device 2610 is coupled to the pad 104of line to be protected, while the drain of the PMOS device 2610 iscoupled to the drain of the NMOS device 2612. The source of the NMOSdevice is coupled to an anode of the first diode D_(s) in the diodechain 320, while the cathode of the last diode in the diode chain 320 iscoupled to ground 112. The gate of the PMOS device 2610 is coupled tothe drain of the PMOS or any lower potential. The gate of the NMOSdevice 2612 is coupled to the drain of the NMOS or any higher potential(e.g., line 2614 drawn in phantom).

During operation, when the temperature of the IC 100 increases, thecurrent through the diodes of the diode chain 320 also increases (i.e.,a negative temperature coefficient). Further, when the temperature ofthe IC 100 increases, the current through the MOS devices 2610 and 2612decreases (i.e., a positive temperature coefficient). As such, the MOSdevices 2610 and 2612 compensate for current increases in the diodechain 320, thereby making the triggering relatively independent of theoperating temperatures. One skilled in the art will understand that thenumber of MOS devices in the temperature compensated trigger device 2608may vary depending on the size and number of diodes in the diode chain320 and on the actual temperature coefficients of the devices used forthe IC 100. Further, the temperature compensated trigger device 2608 maybe utilized at either or both gates G1 334 and G2 336.

FIG. 27 depicts a schematic diagram of a multi-fingered DTSCR ESDprotection device 2702 having current mirrored triggers for each SCRfinger 2706. The DTSCR ESD protection device 2702 comprises atemperature compensated turn-on chain 2708 coupled to a plurality of SCRfingers 2706 ₁ through 2706 _(n), where n illustratively equals 2 (n=2).The multi-fingered DTSCR ESD protection device 2702 is illustrativelycoupled between a supply line VDD 2004 and ground 112. However, oneskilled in the art will recognize that the multi-fingered DTSCR ESDprotection device 2702 may be coupled between any supply line or an I/Opad 104 to be protected.

The temperature compensated turn-on chain 2708 illustratively comprisesa single PMOS device 2610 coupled to three serial diodes forming thediode chain 320, as similarly shown in FIG. 26. The gate of the PMOSdevice 2610 is coupled to the drain. Furthermore, recall that the diodechain 320 acts as a Darlington transistor, where each diode forms astage.

Each SCR finger 2706 comprises an SCR 306 having the anode coupled tothe supply line VDD 2004 and the cathode coupled to ground 112. Further,a PMOS device 2704 is coupled from the supply line VDD 2004 to beprotected, to a trigger gate. For example, the source of PMOS device2704 ₁ is coupled to the supply line 2004, and the drain is coupled tothe first trigger gate G1 336 ₁. The gate and drain of the PMOS device2610 of the temperature compensated turn-on chain 2708 is coupled toeach gate of the PMOS device 2704 of each SCR finger 2706.

During an ESD event at the supply line 2004, the current flowing throughsingle turn-on chain 2708 from the supply line 2004 to ground 112, candrive multiple ESD shunt devices (i.e., SCR fingers 2706) with equaltrigger currents. Additionally, the holding and clamping voltages areheld above the voltage of the supply line 2004, but below theundesirable voltage range 210 of FIG. 2, which may be harmful to thegate oxides of the IC 100. Thus, the trigger currents to each SCR finger2706 are “mirrored” from the current of the turn-on chain 2708. It isnoted that the current mirrors can be set to trigger each or both gatesG1 336 and G2 334 of each SCR finger 2706. It is also noted that themirrored currents are temperature compensated by the temperaturecompensated turn-on chain 708. It is also noted, that a plurality of thesingle turn-on chains 2708 may be placed on the IC 100 connecting to adistributed plurality of SCR fingers 2706. All the gates of the MOSdevices in the turn-on chain and all the gates of the MOS devices in theSCR fingers are coupled. As such, the distributed turn-on chain willsense efficiently an ESD over-voltage condition on the entire IC 100,and will turn-on all SCR fingers 2706 on the IC 100, thereby providing amaximum level of protection.

It is further noted, that the currents may be scaled by the ratio of thesize (length and width) of the MOS transistors 2704 and 2610 such thatthe trigger current to each trigger gate of each SCR finger 2706 areproportional to the current in the turn-on chain 2708. One skilled inthe art will recognize that adding an NMOS device between the diodechain 320 and ground 112, as well as NMOS devices to the second gates G2334 of the SCR fingers 2706, will allow triggering at the second gatesG2 334 of the SCR fingers 2706.

In the embodiments of FIGS. 3-24, the DTSCR device 302 has been usedeither as a power line to ground power line clamp, or as an input/outputto ground clamp. In both cases, the DTSCR device 302 has been used as atwo-terminal structure for shunting current in a single direction,either between the power line 2004 and ground 112, or the I/O pad 104and ground 112. However, an ESD event may occur between any arbitrarypin combination, and the current may have a positive or negativepolarity with respect to a particular pin that is considered groundedduring the ESD event. As such, the SCR 306 may also be used as athree-terminal device, which provides bidirectional ESD protectionbetween the power line 2004 and ground 112, the I/O pad 104 and ground112, and the power line 2004 and the I/O pad 104, as discussed withregards to FIGS. 28-30.

FIG. 28 depicts a schematic diagram of a first embodiment of a SCR 306complementary input protection circuit 2802. The protection circuit 2802comprises a first and second DTSCR 306 ₁ and 306₂ (first and second leg)coupled between the supply line 2004, the I/O pad 104, and ground 112.Referring to the first SCR 306 ₁, the emitter (i.e., anode) of the PNPtransistor 312 ₁ is coupled to the supply line 2004, and the base of thePNP transistor 312 ₁ is coupled to the collector of the NPN transistor310 ₁. The collector of the PNP transistor 312 ₁ is coupled to the firsttrigger gate G1 336 ₁, which is coupled to the base of the NPNtransistor 310 ₁. The emitter (i.e., cathode) of the NPN transistor 310₁ is coupled to the I/O pad 104, and the first trigger gate G1 336 ₁ iscoupled to ground 112.

Referring to the second SCR 306 ₂, the emitter (i.e., anode) of the PNPtransistor 312 ₂ is coupled to the I/O pad 104, and the base of the PNPtransistor 312 ₂ is coupled to the collector of the NPN transistor 310₂. The collector of the PNP transistor 312 ₂ is coupled to the base ofthe NPN transistor 310 ₂, which forms the first trigger gate G1 336 ₂.The emitter (i.e., cathode) of the NPN transistor 310 ₁ is coupled toground 112, and the second trigger gate G2 334 ₂ is coupled to thesupply line 2004.

Diodes are normally added separately to the protection device to providea conductive path for ESD events of the opposite polarity type where theSCR 306 is inactive. However, one skilled in the art will recognize thatsuch additional diodes (i.e., D_(p) and D_(n)) may conveniently be usedas a portion of the SCR's 306 in which they are already present.

During an ESD event, the first SCR 306 ₁ provides a clamp to the supplyline 2004 for a regular stress case where a negative ESD event occurs atthe I/O pad 104 versus the supply line 2004 at ground potential. Thesecond SCR 306 ₂ provides a clamp to ground 112 for a regular stresscase where a positive ESD occurs at the I/O pad 104 versus GND 112 atground potential. The diodes D_(p) and D_(n) for the opposite stresscases (positive ESD at the I/O 104 versus supply 2004 at groundpotential, and negative ESD at the I/O 104 versus GND 112 at groundpotential) are provided by the base-emitter of each SCR 306. During theregular ESD stress cases, one of the base-emitter diodes charges theparasitic VDD-GND capacitance 2804 between supply line 2004 and ground112. In other words, the VDD-GND capacitance 2804 provides an electricload to enable current flow in these base-emitter diodes. When a voltagedrop across the base-emitter diodes at the first gate G1 336 ₁, reachesapproximately plus 0.7 volts, or the second gate G2 334 ₂ reachesapproximately minus 0.7 volts, the SCR's 306 will turn-on and shunt theESD current to the respective ground (i.e., either ground 112 or thesupply line 2004).

FIG. 29 depicts a schematic diagram of a second embodiment of a SCR 306complementary input protection circuit 2902. The second embodiment ofFIG. 29 is the same as the first embodiment of FIG. 28, except that twoadditional diode chains 320 ₁ and 320 ₂ are respectively coupled to thetrigger gates of the SCR's 306 ₁ and 306 ₂. In particular, an anode of afirst diode in a first chain 320 ₁ (illustratively having 3 seriallycoupled diodes) is coupled to the emitter of the PNP transistor 312 ₁,while the cathode of the last diode in the diode chain 320 ₁ is coupledto the first trigger gate G1 336 ₁. Similarly, an anode of a first diodein a second chain 320 ₂ (illustratively having 3 serially coupleddiodes) is coupled to the second trigger gate G2 334 ₂, while thecathode of the last diode in the diode chain 320 ₂ is coupled to theemitter of the NPN transistor 310 ₂.

The first and second diode chains 320 ₁ and 320 ₂ are utilized toprovide a load in addition to the capacitive load of the VDD-GNDcapacitance, and to increase the triggering voltages above the supplyline voltages. Referring also to FIG. 2, the first SCR 306 ₁ willtrigger at approximately 2.8 volts between the I/O pad 104 and ground112. Further, the same analysis may be applied to the second SCR 306 ₂.

FIG. 30 depicts a schematic diagram of a third embodiment of a SCR 306complementary input protection circuit 3002. The third embodiment ofFIG. 30 is the same as the first embodiment of FIG. 28 (or secondembodiment of FIG. 29), except that each leg 3006 ₁ and 3006 ₂ of thecomplementary SCR protection circuit 3002 has a MOS device 3004 as aload element.

In particular, the first SCR leg 3006 ₁ comprises a SCR 306 ₁ having anNMOS device 3004 ₁ coupled in parallel to the NPN transistor 310 ₁, suchthat the source and drain of the NMOS device 3004 ₁ are respectivelycoupled to the emitter and collector of the NPN transistor 310 ₁.Further, the gate of the NMOS device 3004 ₁ is coupled to the firsttrigger gate G1 336 ₁.

Similarly, the second SCR leg 3006 ₂ comprises a SCR 306 ₂ having a PMOSdevice 3004 ₂ coupled in parallel to the PNP transistor 312 ₂, such thatthe source and drain of the PMOS device 3004 ₂ are respectively coupledto the emitter and collector of the PNP transistor 312 ₂. Further, thegate of the PMOS device 3004 ₂ is coupled to the second trigger gate G2334 ₂. The MOS devices 3004 have threshold voltages in a range ofapproximately 0.2 to 0.6 volts, which is less than the respectivebase-emitter or emitter-base junction voltages (i.e., approximately 0.7volts) at the trigger gates G1 336 ₁ and G2 334 ₂ of the SCRs 306 ₁ and306₂.

During a positive ESD event, for example, occurring at the supply line(VDD) 2004, where the I/O pad 104 is at ground potential, the groundline 112 will be pulled up to approximately 0.7 volts from thebase-emitter junction of the NPN transistor 306 ₁. The gate of the NMOSdevice 3004 ₁, which is connected to the first trigger gate G1 336 ₁ ofthe first SCR leg 3006 ₁, has a threshold voltage of less than 0.7V suchthat the NMOS transistor 3004 ₁ will turn on. It is important to notethat the MOS device operates in MOS-mode only, and unlike a prior artdevice such as the low voltage triggering SCR (LVTSCR) having one NMOStriggering device, no breakdown is utilized. Once the NMOS transistor3004 ₁ is turned on, the potential of the trigger gate G2 334 ₁ of theSCR 306 ₁ is pulled low and the SCR is predisposed for conduction. Assoon as the positive ESD voltage at VDD 2004 exceeds the holding voltageof the SCR 306 ₁, the ESD current will be shunted to the grounded I/Opad 104.

During normal circuit operation the GND supply line 112 is grounded suchthat a voltage drop does not appear across the base-emitter of the SCR306 ₁, thereby keeping the gate of the NMOS 3004 ₁ at ground andconsequently, the NMOS device 3004 ₁ turned off. A person skilled in theart will recognize that the same operational analysis applies to thesecond SCR leg 3006 ₂. As such, one benefit of this third embodiment ofFIG. 30 is that there is no leakage current during normal operation, asoccurs with the diode turn-on chain of FIG. 29.

Although various embodiments that incorporate the teachings of thepresent invention have been shown and described in detail herein, thoseskilled in the art can readily devise many other varied embodiments thatstill incorporate these teachings.

1. An electrostatic discharge (ESD) protection circuit in asemiconductor integrated circuit (IC) having protected circuitry, theESD protection circuit comprising: a pad adapted for connection to afirst voltage source of a protected circuit node of the IC; a siliconcontrolled rectifier (SCR) having an anode adapted for coupling to thefirst voltage source, and a cathode adapted for coupling to a secondvoltage source; and at least one capacitive turn-on device having afirst and a second terminal such that the first terminal is directlyconnected to a second gate of said SCR and the second terminal isdirectly connected to the second voltage source, wherein said capacitiveturn-on device is not part of the SCR and is adapted to provide atrigger current directly to the SCR.
 2. The ESD circuit of claim 1further comprises a first capacitor coupled between said first voltagesource and said first gate of said SCR.
 3. The ESD circuit of claim 2,wherein said at least one capacitive turn-on device further comprises asecond-capacitor coupled between said second gate of said SCR and saidsecond voltage source.
 4. The ESD circuit of claim 1, wherein: saidanode comprises a P-type anode formed in an N-well bulk region; and saidcathode comprises an N-type cathode formed in a P-well bulk region. 5.The ESD circuit of claim 4, further comprising: at least one series PNjunction diode, each series diode having an anode and a cathoderespectively coupled in a forward conduction direction from a secondgate of the SCR towards ground, wherein the P-type anode and the N-wellbulk region of said SCR form a second PN junction diode, said second PNjunction diode being serially coupled in a forward direction with saidat least one series PN junction diode.
 6. The ESD circuit of claim 4,further comprising: at least one series PN junction diode, each seriesdiode having an anode and a cathode respectively coupled in a forwardconduction direction from said pad towards a first gate of the SCR,wherein the P-well bulk region and the N-type cathode of said SCR form asecond PN junction diode, said second PN junction diode being seriallycoupled in a forward direction with said at least one series PN junctiondiode.
 7. The ESD protection circuit of claim 1, wherein said at leastone capacitive turn-on device comprises a capacitance value in a rangeincluding one pico-Farads (1 pF) to one nano-Farads (1 nF).
 8. The ESDcircuit of claim 1, wherein said first voltage source is a positivevoltage source.
 9. The ESD circuit of claim 1, wherein said secondvoltage source is ground.
 10. The ESD circuit of claim 1, furthercomprising: a first resistor coupled to said first gate of the SCR andsaid second voltage source.
 11. The ESD circuit of claim 10, wherein thefirst resistor comprises the intrinsic resistance of said SCR.
 12. Anelectrostatic discharge (ESD) protection circuit in a semiconductorintegrated circuit (IC) having protected circuitry, the ESD protectioncircuit comprising: a pad, adapted for connection to a protected circuitnode of the IC; a silicon controlled rectifier (SCR), coupled betweenthe pad and ground; a first resistor, coupled to a first gate of the SCRand ground; a power voltage source; and a coupling capacitor having afirst and a second terminal such that the first terminal is directlyconnected to a second gate of said SCR and to the power voltage source,and the second terminal is directly connected to the ground, whereinsaid coupling capacitor is not part of the SCR and is adapted to providea trigger current directly to the SCR.
 13. The ESD protection circuit ofclaim 12, further comprising a second coupling capacitor coupled to thefirst gate of said SCR and the pad.
 14. The ESD protection circuit ofclaim 12 further comprising a second resistor coupled to the second gateof the SCR and the power voltage source.
 15. An electrostatic discharge(ESD) protection circuit in a semiconductor integrated circuit (IC)having protected circuitry, the ESD protection circuit comprising: a padadapted for connection to a first voltage source of a protected circuitnode of the IC; a silicon controlled rectifier (SCR) having an anodeadapted for coupling to the first voltage source, and a cathode adaptedfor coupling to a second voltage source; at least one capacitive turn-ondevice respectively coupled between at least one of a first gate of saidSCR and the first voltage source, and a second gate of said SCR and thesecond voltage source, wherein said capacitive turn-on device is notpart of the SCR; and at least one diode coupled in the forwardconduction direction from said capacitive turn-on device to said firstgate of said SCR when said capacitive turn-on device is coupled betweenthe first gate of the SCR and the first voltage source.
 16. Anelectrostatic discharge (ESD) protection circuit in a semiconductorintegrated circuit (IC) having protected circuitry, the ESD protectioncircuit comprising: a pad adapted for connection to a first voltagesource of a protected circuit node of the IC; a silicon controlledrectifier (SCR) having an anode adapted for coupling to the firstvoltage source, and a cathode adapted for coupling to a second voltagesource; at least one capacitive turn-on device respectively coupledbetween at least one of a first gate of said SCR and the first voltagesource, and a second gate of said SCR and the second voltage source,wherein said capacitive turn-on device is not part of the SCR; and atleast one diode coupled in the forward conduction direction from saidsecond gate of said SCR to said capacitive turn-on device when saidcapacitive turn-on device is coupled between the second gate of the SCRand the second voltage source.
 17. The ESD circuit of claim 16 whereinsaid diode comprise at least one series PN junction diode, each seriesdiode having an anode and a cathode respectively coupled in a forwardconduction direction from the second gate of the SCR towards ground,wherein the P-type anode and the N-well bulk region of said SCR form asecond PN junction diode, said second PN junction diode being seriallycoupled in a forward direction with said at least one series PN junctiondiode.
 18. The ESD circuit of claim 15, wherein said diode comprises atleast one series PN junction diode, each series diode having an anodeand a cathode respectively coupled in a forward conduction directionfrom said pad towards a first gate of the SCR, wherein the P-well bulkregion and the N-type cathode of said SCR form a second PN junctiondiode, said second PN junction diode being serially coupled in a forwarddirection with said at least one series PN junction diode.